1. Field of the Invention
The present invention pertains to an uninterruptible power supply system equipped with a rechargeable and dischargeable lithium-ion secondary battery acting as a backup power source of a power supply unit having a main power source unit built-in.
2. Related Prior Art
In general, when an input voltage from a commercial power source is normally supplied, this sort of the uninterruptible power supply system supplies necessary direct currents or alternate currents from the main power source of the power supply unit to a load with a secondary battery being charged through the main power source. In contrast, when an input voltage has been reduced seriously or power interruption has occurred, the secondary battery acting as the backup power source discharges to keep on its power supply. As the secondary battery employed here, a lead battery has been hitherto employed due mainly to its low price and easiness in the management of charging and discharging. There are, however, problems that the lead battery requires a larger housing space and is extremely heavy. Further, various environmental problems of lead contamination are unavoidable. For solving these problems, Japanese unexamined application publication No. 2002-58170 proposes an uninterruptible power supply system that employs a lithium-ion secondary battery instead of the lead battery as a backup power source. The lithium-ion secondary battery employed here comprises battery modules consisting of a plurality of series-connected battery cells. A battery protection circuit for preventing overcharge and over discharge is also built in this uninterruptible power supply system proposed.
According to the above-mentioned conventional uninterruptible power supply system, a backup time to keep on power supply at the time of voltage reduction or power interruption depends on the service (or discharge) capacity of the secondary battery. This service capacity, however, lowers gradually as the secondary battery is used over a long period of time. FIG. 12 shows respective characteristic of charging voltage V1 and service capacity P1 during the duration of service of the secondary battery. Heretofore, the service capacity sustainable at the end of life of the secondary battery has been estimated and then the charging voltage V1 supplied from the main power source to the secondary battery has been set so as to be able to sustain the service capacity P1 enough to ensure at least the backup time required for the system even at the end of life of the secondary battery.
However, in reality, the charging voltage V1 has heretofore been set at a fixed value in the main power source regardless of how long the secondary battery has been operated. Hence, in early phase of the life of the secondary battery, charging is carried out with the service capacity P1 of the secondary battery being more than enough for the required backup time (see “excessive setting” shown in FIG. 12). Stated differently, since the charging voltage V1 applied to the secondary battery is preliminarily set at a higher value than actually needed in expectation of deterioration with age in the secondary battery, the lithium-ion secondary battery whose life is markedly shortened when overcharged in particular has a problem that the life of the secondary battery, eventually that of the uninterruptible power supply system is seriously shortened.